1. Field of the Invention
The present invention relates generally to aerodynamic flight control of aircraft and relates specifically to passive jet spoilers used to create yaw moments.
2. Description of the Related Art
Wings for aircraft are constructed to have a longitudinal profile that produces lift as air flows across the upper and lower surfaces of the wing. The air flows with a greater velocity over the upper surface, producing a low-pressure area, and the high pressure below the wing pushes the wing upward. Ideally, the air flows across the wing in an attached, laminar stream adjacent a low velocity boundary layer, minimizing inherent drag and maximizing lift of the wing for a given airspeed. When the boundary layer becomes separated, or detached, from a portion of the wing, the flow becomes turbulent, and the wing is said to “stall” in that area. This increases drag and reduces lift.
It is known in the art that this separation can be caused by various means, including solid and jet spoilers. A solid spoiler is typically an articulating element that is moved from a streamlined position into a position in the airflow that causes the separation. A jet spoiler produces the separation by injecting pressurized air through holes or slots in the wing for separating the boundary layer. One advantage of jet spoilers over solid spoilers is the reduced radar signature, since no solid elements are moved into the airflow. Prior-art jet spoilers have typically included a source of pressurized air provided by the engines or other source carried on the aircraft, and this type of spoiler is referred to herein as an “active” spoiler.
Jet spoilers were studied in the past for producing roll moments, particularly for missiles, though yaw moments were considered undesirable. However, as described in “The Jet Spoiler as Yaw Control Device,” AIAA 86-1806, D. A. Tavella, et al., conducted experimentation using active jet spoilers for aircraft wings to produce yaw moments. This research built on a concept for a “passive” jet spoiler disclosed in U.S. Pat. No. 4,504,192 to Cyrus, et al., for controlling the power output of a wind-powered Darrieus turbine. The blades of the turbine are hollow, and air within the blades is accelerated outward as the turbine rotates. By providing an inlet at an inner end of the blades and holes on the inner and outer surfaces of the blades, air is drawn into the inlets and blows out of the outlets. This air disrupts the airflow across the surfaces of the blades, producing drag on the blades for limiting the speed of rotation or for use in an emergency shutdown.
The tests by Tavella, et al., used a slot in a test wing to inject high-velocity, pressurized air into the boundary layer on the upper surface only or at opposing positions on the upper and lower surfaces, the positions being located approximately the same distance from the leading edge of the wing. While effective for producing yaw moments, a significant roll moment was also produced.
There is a need for a passive jet spoiler for yaw control of aircraft, with the spoiler having a maximized yaw moment and a minimized roll and pitch moments. Also, there is a need for a passive jet spoiler that uses the air flowing on the underside of the wing for providing the air to the outlets for separating the airflows. Additionally, there is a need for a passive spoiler having valves for controlling the amount of air passing through the spoiler and/or the relative amounts of air flowing out of multiple outlets.